Large clinical trials have shown a reduced incidence of type 2 diabetes mellitus (T2DM) in postmenopausal women randomized to estrogen-based hormone therapy (HT) compared to placebo. Moreover, a recent population-based prospective cohort study demonstrated development of T2DM was 69% lower in postmenopausal women who had used HT for more than half of a 5-yr follow-up period compared to women who never used HT. Consistent with this, our preliminary data suggest that the timing of estrogen treatment relative to the menopause may be an important determinant of whether there are favorable effects on insulin action. We observed an inverse association of years since menopause with estradiol (E2)-mediated improvements in insulin-stimulated glucose disposal rate (GDR), such that E2 improved GDR in early postmenopausal women, but decreased GDR in those more than 10 years past menopause. Accumulating data suggest estrogens have divergent effects on cardiovascular risk when initiated close to the onset of menopause rather than distant from the menopause. We hypothesize this is also true for insulin action. The general aim of the current proposal is to determine the effects of time since menopause and duration of estrogen deficiency on the ability of E2 to improve insulin action. As such, we propose to measure GDR (via hyperinsulinemic-euglycemic clamp) in women who are 6 years of the onset of menopause (earlier post menopausal;EPM) or 10 years beyond the menopause (later post menopausal;LPM) and who are naive to HT. All women will be studied with and without short-term (2 weeks) administration of transdermal E2 in a randomized, cross-over design. Our global hypothesis is that the E2-mediated effects on insulin action depend on duration of estrogen deficiency. We postulate that E2 will increase GDR in early postmenopausal women and decrease GDR in late postmenopausal women. Because altered estrogen receptor (ER) expression after prolonged estrogen deficiency could account for reduced ability of E2 to augment insulin action, as an exploratory outcome we will compare ER expression in skeletal muscle and adipose tissue between groups and in response to E2. Confirmation of our hypotheses will provide evidence for a benefit of E2 on insulin action (GDR) when administered early, but not late, after menopause;likely contributing to delayed onset of T2DM. PUBLIC HEALTH RELEVANCE: One of the reported benefits of estrogen therapy in postmenopausal women is reduced incidence of diabetes, but the reason for this is unknown. This study is designed to test whether estrogens improve insulin sensitivity and whether this effect depends on how long after menopause treatment is initiated. It is expected that estrogens improve insulin sensitivity when given early, but not late, after menopause;likely delaying the onset of diabetes.